Hippocampus and Memory: Neuroanatomy and Clinical Disorders

Overview

The hippocampus is a curved structure in the medial temporal lobe, named for its resemblance to a seahorse (Greek: hippos = horse, kampos = sea monster). It is critical for consolidating new declarative memories — facts (semantic) and events (episodic) — and spatial navigation.

Anatomical Location and Composition

Located in the floor of the inferior horn of the lateral ventricle, the hippocampal formation consists of:

Hippocampus proper (Ammon's horn, cornu ammonis)
Dentate gyrus
Subiculum
Entorhinal cortex (the major gateway from neocortex)

Cytoarchitecture: Cornu Ammonis Layers

The hippocampus has a characteristic layered structure:

CA1: Most vulnerable to ischemia (Sommer sector). First region damaged in global hypoxia.
CA2: Relatively ischemia-resistant.
CA3: Receives mossy fiber input from dentate granule cells.
CA4: Within the hilus of the dentate gyrus.

The Trisynaptic Circuit

Information flow through the hippocampus follows a classic three-step pathway:

Perforant path: Entorhinal cortex → dentate gyrus granule cells
Mossy fiber pathway: Dentate granule cells → CA3 pyramidal cells
Schaffer collateral pathway: CA3 → CA1 pyramidal cells → subiculum → back to entorhinal cortex

This unidirectional loop is the anatomical basis for memory encoding and retrieval.

Long-Term Potentiation (LTP)

The cellular mechanism of memory: high-frequency stimulation of CA3 axons strengthens synaptic transmission at Schaffer collateral-CA1 synapses for hours to days. NMDA receptor activation (with co-incident glutamate + postsynaptic depolarization) allows Ca²⁺ influx, triggering AMPA receptor insertion and structural synaptic changes.

Clinical Correlations

Alzheimer's Disease

Earliest and most severe neurodegeneration occurs in the entorhinal cortex and CA1
Results in anterograde amnesia (inability to form new memories) — patients cannot remember what they ate for breakfast
Neurofibrillary tangles (hyperphosphorylated tau) and amyloid plaques accumulate
Atrophy visible on MRI as hippocampal volume loss with widened temporal horns

Temporal Lobe Epilepsy

Hippocampal sclerosis (gliosis + neuronal loss, especially in CA1) is the most common pathological finding
Complex partial seizures may produce déjà vu, olfactory hallucinations, or staring spells
Often refractory to medication — selective amygdalohippocampectomy can be curative

Transient Global Amnesia

Sudden onset of anterograde amnesia lasting up to 24 hours
Thought to involve transient hippocampal dysfunction, possibly venous ischemia
Patients repeat the same questions ("Where am I? What day is it?") but retain identity

H.M. (Henry Molaison)

Famous case: bilateral medial temporal lobectomy for epilepsy in 1953
Resulted in profound anterograde amnesia with preserved procedural memory and working memory
Established the hippocampus as the memory consolidation center

Functional Imaging

Functional MRI studies show hippocampal activation during:

Spatial navigation (place cells discovered by O'Keefe — Nobel Prize 2014)
Episodic memory encoding and retrieval
Pattern separation (dentate gyrus distinguishes similar memories)

Key Takeaway

The hippocampus is the brain's memory hub. CA1 is its Achilles heel — vulnerable to ischemia, Alzheimer's, and epileptic damage. Preserving hippocampal function is a central goal in neurology and geriatrics.